In sample observation using an electron microscope, a so-called in-situ observation is sometimes performed for dynamically observing in situ a process of reaction between a sample in a gaseous atmosphere and the gas. With regard to a fuel cell catalyst, for example, exposure of the catalyst to the gas induces migration of noble metal particles on a carrier, resulting in the growth of noble metal particles. A degradation analysis of the catalyst is made by observing in situ the growth of the particles.
This type of observation dictates the need for dividing a primary space in a lens barrel maintained in a high vacuum state from a space in the vicinity of a sample which is maintained in a low vacuum state with a gas. The dividing method is broadly classified into a partition wall type and a differential exhaust type. As a technique related to the former method, Japanese Unexamined Patent Application Publication No. 2003-187735 discloses a sample holder which hermetically seals a sample in a gaseous atmosphere. This sample holder includes: a sample placement portion formed with an opening for allowing the passage of an electron beam; a heating wire (sample heater) so extended as to intersect the opening; a partition wall for isolating the sample placement portion from the interior of a sample chamber (vacuum); and a gas inlet tube (gas inlet device) for introducing a gas into a sample placement space defined by the partition wall.